Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L). This means that every kilogram (roughly one liter by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na+) and chloride (Cl−) ions). A clear change in salinity has been detected in the world's oceans, signaling shifts and acceleration in the global rainfall and evaporation cycle tied directly to climate change. In a paper published Friday (April 27) in the journal Science, Australian scientists from the Commonwealth Scientific and Industrial Research Organization (CSIRO) and Lawrence Livermore National Laboratory reported changing patterns of salinity in the global ocean during the past 50 years, marking a clear symptom of climate change.

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Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in unusually salty water. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater still.

Lead author Paul Durack said that by looking at observed ocean salinity changes and the relationship between salinity, rainfall and evaporation in climate models, they determined the water cycle has become 4 percent stronger from 1950-2000. This is twice the response projected by current generation global climate models.

"These changes suggest that arid regions have become drier and high rainfall regions have become wetter in response to observed global warming," said Durack, a post-doctoral fellow at Lawrence Livermore National Laboratory.

Scientists monitor salinity changes in the world's oceans to determine where rainfall has increased or decreased.

With a projected temperature rise of 3 degrees Celsius by the end of the century, the researchers estimate a 24 percent acceleration of the water cycle is possible.

Scientists have struggled to determine coherent estimates of water cycle changes from land-based data because surface observations of rainfall and evaporation are sparse. According to the team, global oceans provide a much clearer picture.

"Warming of the Earth's surface and lower atmosphere is expected to strengthen the water cycle largely driven by the ability of warmer air to hold and redistribute more moisture."

He said the intensification is an enhancement in the patterns of exchange between evaporation and rainfall, and with oceans accounting for 71 percent of the global surface area, the change is clearly represented in ocean surface salinity patterns.

In the study, the scientists combined 50-year observed global surface salinity changes with changes from global climate models and found "robust evidence of an intensified global water cycle at a rate of about 8 percent per degree of surface warming," Durack said.

Durack said the patterns are not uniform, with regional variations agreeing with the "rich get richer" mechanism, where wet regions get wetter and dry regions drier.

He said a change in freshwater availability in response to climate change poses a more significant risk to human societies and ecosystems than warming alone.